What determines a welder's OCV?

What determines a welder's OCV? Is it a characteristic of the transformer or does it depend on the electronics the manufacturer builds into the welder's circuitry? Is it expensive to build a stick welder with high OCV? It seems like it must be or why would any manufacturer bother to produce welders that were hard to use and didn't work well? After all, word gets around.

Stan

I see where your idea to select a machine on OCV might be a benefit on spec's on high end machines but one would still have to revert back to the first important basics of duty cycle and amperage as a place to start when comparring spec's of different machines.

The reason I say this, is on most CC machines most designs are based on 80/OCV and most CV machines are designed on 45/OCV from what I can find in my text refferences. That may very well be outdated info on the inverter based designs.

To me it still boils down too finding a machine with the correct duty cycle for the amount of amps I want to run. Then it's off to the Vo-Tech schools to try the lastest and greatest machines as most Vo-Techs run machines that are not very old, less than 3yrs old, or the machine technology hasn't changed.

Back when, we would send a couple experienced weldors to the welding supply shop to try out the machines before we bought them. I've asked about that at my local supply shops and they told me they don't do that any more.

As to the machines starting ability, I haven't used any of the machines less than a $1000 that has a hard starting arc. That's probably due to my technique of using approx 150a on 1/8" electrodes depending on the metal thickness. I just don't see a problem from here.

The Miller Thunderbolt 200aAC/150aDC, about $425, that I bought my son 4yrs ago had a weld puddle that closely matched a AC molten puddle while on DCEP using 7018. I contribute that to the cheap transformer design with little external arc stability componets found in high end machines. I'm not saying it wasn't a adequate arc for the price of the machines today but it sure wasn't even close to the old Lincoln 225a AC arc of 50yrs past.

For machines under $500, a person is just not going to find a machine with more than a 20% duty cycle at around 150a, so to me OCV would be a mute spec point on the cheaper machines and not much more useful on the high end machines as OCV is taken from terminals at the machine and not at the point of use.

T_Bone

It's almost totally a matter of curiosity for me now---something I'd like to understand better, but with no specific application in mind at this time. Basic information has a way of being useful to you unexpectedly, though. Learn enough of the basics about a complex system and it will sometimes start to make sense in a way that will allow you to deal with it more usefully.

What I've learned about OCV just from this thread has made me see that, taken alone, it's not going to provide the simple benchmark of arc starting capabiblity in low end machines that I hoped it would.

I thought you might be interested in what the experts at Lincoln Electric had to say about OCV when I asked the same question there:

"Good Morning Stan,

It is a design characteristic determined by the engineers when the manufacture the machine."

That's it, in its entirety. Of course, it was free.

All the best, Stan

I know that you can measure the OCV by putting your voltmeter across the leads, because I've done that. I'm not sure whether you can measure the working voltage across the leads while welding, but I'll try that tonight and let you know.

BTW, I've been meaning to ask you, are you the same Fred who used to use Fred OH as your handle, and sign off with "L8R, Fred"? I liked that Fred, too.

All the best, Stan

The typical power supply type transformer has a core size and materials such that the core never saturates, i.e. never reaches the limit of its ability to be magnetized during each half cycle of the AC input. In this case, the open circuit voltage is determined solely by the ratio of the windings, and the loaded output voltage is the open circuit voltage minus the voltage drop due to the output current flowing through the resistance of the secondary windings.

However, if the core is designed to modify the way it is magnetized during each half cycle, then the transformer can produce an almost constant output current, and will vary the output voltage to force this current through whatever load is applied, up to the maximum voltage as determined by the primary/secondary windings ratio. This is the typical approach to an AC buzzbox welder. By selecting the turns ratio to produce a high open circuit voltage, and then depending on the core characteristics to regulate the current, I can make the welder easy to start.

Now, in order to save money, the manufacturer would want to build the transformer with the least number of secondary turns and the smallest possible wire size. By reducing the number of secondary turns to the minimum that can produce enough voltage to sustain an arc, the manufacturer reduces the amount of material required and the cost of manufacturing the transformer, without having to give up any of the rated output current.

However, by reducing the number of turns, the manufacturer does reduce the maximum open circuit voltage, which is determined by the primary/secondary turns ratio. This results in the lowest cost for a given welding current, but makes starting the arc difficult.

Switching mode (inverter) supplies also use a transformer, but run it at frequencies in the 10's to 100's of kiloHertz, rather than at the 60 Hz power line rate. The high frequency input is generated by rectifying the 60 Hz input and then using an electronic switch to "chop" the DC into the high frequency pulses applied to the transformer. The output voltage is regulated by controlling the duty cycle of the chopper, which in turn controls the average current into the transformer.

If I were designing a welder based on inverter technology, I would include an active voltage regulator that would raise the voltage to start the arc, and then lower it to whatever is needed to maintain the desired current. I've never looked at the circuit for one, but I'd suspect that the cheapo Chicago Electric on probably omitted the active voltage regulation and simply controls the chopper to a single voltage, hence the low open circuit voltage.

Keith

Stan

If it is a newer one without transformer, the electronics determines the OCV.

As far as the best OCV I think it sort of depends on what you are trying to do with the wleder, ie rod size, etc and how well the voltage holds up after you strike the arc, but I will leave that to the more expert like Tbone and such.